Jon gives us good technically-based answers, so starting with addressing his advice/suggestions is appropriate.
I have used a variety of carburetor(s) on my 1960 261 (bored 0.060 over - it is a little under 270ci).
After reading some of Jon's posts years ago, I opted for my "lame logic" and looked for carburetor/carburetors that matched the cubic inches (CFM might have been a better measure, but that is not easily measured after a over-bore, I guessed).
I now have settled on one Holley AA1 two-barrel carburetor from a GMC 302, with two adjustable jets. Maybe a GMC 270's Zenith (?) one barrel carburetor (with an adjustable) would be as good, or better. A vacuum port had to be drilled into it, and I had to find an intake manifold adapter plate (Larrowe & Sons, I think).
My 261 has never driven more smoothly and with as much power (all empirical "measurements" of performance). Come to eastern NC, and I'll take you for a ride.
